#include<iostream>
using namespace std;
int digui(int n)
{
	if (n == 1)
		return 1;
	else if (n == 2)
		return 2;
	else
		return digui(n - 1) + digui(n - 2);
}
int main()
{
	int s[21];
	int x[21];
	
	for (int i = 1; i <= 20; i++)
	{
		s[i] = digui(i);
		x[i] = digui(i + 1);
	}
	double res = 0;
	for (int i = 1; i <= 20; i++)
	{
		res += double(s[i])/ x[i];
	}
	cout << res;

	return 0;
}


